The present invention generally relates to methods of forming two-sided prints, and more particularly to a method of forming a two-sided print which is applicable to an image forming apparatus such as a laser printer, a copying machine and a facsimile machine.
Conventionally, there is a method of forming a two-sided print in the following manner. That is, a reversing unit and an image forming unit are provided in a transport path of a recording sheet. First, the image forming unit forms an image on a first side of the recording sheet which is supplied from a paper supplying unit. Then, this recording sheet is turned over, that is, reversed in the reversing unit so that the image forming unit can form an image on a second side of the recording sheet.
For example, when copying 60 pages of a document onto 30 two-sided copies on a copying machine, 30 pages of the document are first copied onto the first sides of 30 recording sheets by feeding the 30 recording sheets to the image forming unit. The 30 recording sheets having images copied on the first sides thereof are stacked on an intermediate tray. Thereafter, the remaining 30 pages of the document are copied on the second sides of the 30 recording sheets by successively refeeding the 30 stacked recording sheets from the intermediate tray to the image forming unit. A two-sided print forming mode employing this method is often referred to as a stack mode.
However, according to the conventional stack mode, the stacked recording sheets on the intermediate tray must be separated one by one by a separating unit when successively refeeding the stacked recording sheets to the image forming unit. For this reason, when a malfunction of the separating unit occurs, there is a problem in that the reliability or quality of the paper transport deteriorates. In other words, the malfunction of the separating unit may cause two recording sheets to be refed to the image forming unit one on top of the other.
On the other hand, the alignment of the recording sheets which are stacked on the intermediate tray is made by adjustable guide plates which guide the two opposite sides of the recording sheets. But because the stacked recording sheets are aligned by the two opposite side edges thereof, it is impossible to stack recording sheets of different sizes in alignment. Accordingly, there is a problem in that the two-sided prints cannot be made successively on the recording sheets of different sizes.
Furthermore, a paper jam may occur when making the two-sided prints. In this case, it is necessary to obtain an accurate information on the page which is to be printed when the printing is resumed and the image which is lost if any due to the paper jam. Unless the accurate information is obtained, there is a problem in that the operation of making the two-sided prints must be repeated from the beginning.
On the other hand, a delay is introduced in the timing of the image in units of jobs or pages when a delay occurs in an image processing which is carried out by a controller. When such a timing delay of the image occurs, the conventional image forming apparatus does not wait for the delay to be corrected. In other words, the conventional image forming apparatus does not print an image and simply ejects a blank recording sheet. For this reason, there is a problem in that the user must afterwards make an appropriate editing of the prints.